1. Field of the Invention
The present invention relates to a liquid crystal display unit and a method for manufacturing the same, in particular to a smoothening film without occurrence of abnormal orientation of liquid crystal molecules and a method for manufacturing the same.
2. Description of the Prior Art
For the downsizing of a liquid crystal projector, it is advantageous to use a poly-silicon thin film transistor (Poly-Si TFT) as an active element of a liquid crystal panel. This is because the poly-Si TFT can provide sufficient characteristics even with a TFT element smaller size and further a peripheral drive circuit can also be integrated in the liquid crystal panel.
The performances required for a liquid crystal projector include the size and the luminosity of a project screen. To enlarge the project screen and further raise the luminosity, it is necessary to irradiate an intense light on the liquid crystal panel. When light is irradiated on poly-Si TFT elements disposed in individual pixels of the liquid crystal panel, however, an optical leakage current is generated and the voltage applied to each pixel fluctuates. As a result, the image quality significantly deteriorates. Besides, not only light irradiated to the liquid crystal panel from the light source but reflected light generated in the process of transmission through an optical system such as lenses after the transmission through the liquid crystal panel also produces an optical leakage current similarly, thereby deteriorating the image quality significantly.
Accordingly, to protect the TFT from these rays bringing about optical leakage currents, there is often used such a structure that the top and the bottom of a TFT are covered with a metal film (referred to as light shielding film) so as to prevent light from hitting on the TFT.
FIG. 1 is a sectional view of the TFT part whose top and bottom are covered with a metal film in a conventional liquid crystal display unit. This conventional liquid crystal display unit comprises a substrate with a TFT of an active element formed thereon (referred to a TFT substrate 3), a substrate with a common electrode formed thereon (referred to as opposite substrates 4) and a liquid crystal layer 615 packed between these substrates.
The TFT substrate 3 with a transparent substrate 600 made of glass or the like employed as the base substrate, comprises a lower light shielding film 601, an underlying film of insulating nature 602, a poly-Si film 619, a gate insulating film 605 made of SiO2 or the like and a gate electrode 606 formed in sequence on this substrate 600. Furthermore, the TFT substrate 3 comprises the first inter-layer insulating film 607, a wiring metal film 608, the second inter-layer insulating film 609, an upper light shielding film 610, the third inter-layer insulating film 611, a transparent electrode 613 in use for pixel electrodes, made of an ITO film (Indium Tin Oxide film), and an alignment layer 614 made of a polyimide resin or the like formed in sequence on the gate insulating film 605.
Into part of the poly-Si film 619, impurities are injected to form a drain region 603 and a source region 604 and the intersection part of these poly-Si film 619 and gate electrode 606 serves as a TFT.
On the other hand, the opposite substrate 4 with a transparent substrate 618 made of glass or the like employed as the base substrate, comprises a transparent electrode 617 in use for the common electrode, made of an ITO film (Indium Tin Oxide film), and an alignment layer 616 made of a polyimide resin or the like formed in sequence on this base substrate.
In order to solve a problem of light leakage current, the TFT substrate 3 shown in FIG. 1 comprises numerous layers as mentioned above. On the surface of the alignment layer 614 at the top of the TFT substrate 3, a ruggedness is generated depending on the patterning of the lower layer. The step difference due to this ruggedness may amount to 1 xcexcm or more and is a negligible value relative to the thickness (3 to 5 xcexcm) of the liquid crystal layer. Thereby, a disorder in the orientation condition of liquid crystal molecules called reverse tilt or reverse twist occurs, thus deteriorating the display image quality significantly.
To solve this problem, means for forming a coat film of an organic or inorganic material below the transparent electrode of a TFT substrate to smooth the ruggedness is effective. In Japanese Patent Application Laid-Open No. 10-90669, an example of using a photosensitive acrylic resin is disclosed as a film of performing this smoothening (hereinafter, referred to as smoothening film).
Here, it is regarded as preferable to use an acrylic resin transparent to visible light as a base polymer, such as epoxy acrylate or urethane acrylate. As such an acrylic resin, a negative- or positive-type acrylic resin with a photosensitive group absorbing UV rays, of a wavelength not greater than 380 nm, is used so as to prevent the smoothening film from being discolored under action of visible light and keep the transmittance of the panel from lowering.
In the technique of the above publication, the photosensitive group remaining after the formation of a contact hole in the smoothening film of an acrylic resin is allowed to be decomposed under action of a high energy UV ray, but is difficult to completely decompose.
On the other hand, a high luminance light source has been used to improve the screen luminance of a projector and UV rays of 300 to 380 nm wavelength, contained in the irradiated light of a high luminance light source, to be originally cut off before entering a liquid crystal panel transmit through the filter even in a small amount to reach the liquid crystal panel. For this reason, according to the technique of the above publication, the photosensitive group left not completely decomposed in the smoothening film made of a photosensitive acrylic resin absorbs the UV rays of 300 to 380 nm wavelength leaking and reaching the liquid crystal panel. As a result, there were problems that the smoothening film was discolored and bubbles were generated in the smoothening film to reduce its transmittance in the visible light range. Beside, the technique of the above publication had fear of deterioration of the smoothening film (acrylic resin) itself under action of high energy UV rays to be used for the decomposition of the residual photosensitive group.
Hence, it is an object of the present invention to provide a liquid crystal display unit used for a liquid crystal projector and having a smoothening film without occurrence of an abnormal orientation of liquid crystal molecules and deterioration of the image quality as well as a method for manufacturing the same.
With the present invention, light shielding films are disposed at least on one side above and below the TFT of a TFT substrate and a transparent acrylic resin not absorbing ray of not smaller than 300 nm wavelength, formed by the thermal polymerization, is used to form a smoothening film below the transparent electrode of the TFT substrate. With the present invention, this structure prevents the smoothening film from being denaturalized or discolored even under action of the UV rays of not smaller than 300 nm wavelength of an intense light source such as projector, leaking from a filter. Besides, since a smoothening film is formed by the thermal polymerization, no photosensitive group absorbing a ray of not smaller than 300 nm wavelength is also present in the smoothening film. Accordingly, there is no need for irradiation treatment using a high energy UV ray after the film formation unlike the prior art and the smoothening film is not damaged by a high energy UV ray to generate bubbles.
Besides, with the present invention, a ray is incident to a TFT so as not to deteriorate the image quality and further the ruggedness generated by stacking a plurality of light shielding films is moderated by the smoothening film and no abnormal orientation of liquid crystal molecules occurs. Thereby, occurrence of a reverse twist or reverse tilt is prevented.